not talking much anymore

when my babies were around 10-11 months they could say a handful of words... dada, dog, baba, bye bye, night, book... now they are almost 15 months and only says Dada and dog?? Is this from learning to walk and dropping talking or something (they've been walking for almost 3 months and are very good at it)? they won't repeat words but will look or grab at what I say. Is there anything else I can do to help them?